1. Field of the Invention
The invention relates to a magnetic resonance spectrometer in which an analog-to-digital converter in the receiving branch converts the spin resonance signals, or a spin resonance signal transposed to a lower frequency range, into a digital signal, and in which a Fourier transformation circuit is arranged in a signal path subsequent to the analog-to-digital converter.
2. Description of the Prior Art
A magnetic resonance spectrometer of this kind is described in DE-OS 36 04 281 corresponding to commonly owned U.S. Pat. No. 4,733,184. Therein the spin resonance signal is first converted to the baseband after which it is converted into a series of digital data words by means of an analog-to-digital converter. The conversion to the baseband necessitates quadrature demodulation, the spin resonance signal then being mixed with an oscillation having the Larmor frequency which is applied to the two mixing stages with a 90.degree. phase shift.
The construction of the two mixing stages of the known spectrometer must be exactly symmetrical and the phase shift between the two oscillations must be exactly 90.degree.. In an analog mixing stage, it is rather complex to satisfy this requirement. A further drawback consists in that the lowest useful frequency in the baseband is so low that a dc amplifier is required for amplifying the output signal of the mixing stages. A shift of the operating point of this amplifier causes a disturbing drift which gives rise to reconstruction errors during the further processing of the signal. Moreover, the low-frequency noise (also referred to as flicker noise) in the baseband and the harmonics of the mains frequency have a particularly disturbing effect.